How to insulate a vaulted ceiling
I am an American living in Provence France. We have hot summers and for the most part mild winters. In January, temperature can drop into the upper twenty during the night. It is a dry climate. I have been renovating a house at different times since last April. During the spring of the year I did the outside roof structure. I installed a new wood decking then a product called flexotuille designed to put under a clay tile roof. After a few other project, I have removed the inside ceiling in the den area in order to vault the ceiling in this area. Some of the structural beams where failing and needed to be replaced. The plan is to leave all structural beams exposed and insulate between the rafters which are 15 cm deep. The heat source in the den is a very efficient wood burning stove and no A/C. After reading Martin Holladay article for insulating vault ceilings, l have a concern. In the winter, with warm inside air passing through the insulation and coming in contact with cold air passing through the roof structure. Will this cause condensation with a potential mold problem? All the insulation contractors said this is not a problem and the roof will dry to the outside. I don’t really believe this b/c there is no air flow. A vented roof system is not possible. I am sure closed cell foam is the best option but does fit into the budget after all the other work. Here is some different option. The first is mine and the others are the insulation contractors. They thought my idea for the panels was crazy.
Can rigid foam XPS panels go between the rafters? I have found a panel that is just a little wider than the space between the rafters. Can the panels touch the bottom of the sheathing? What is the minimum thickness the panels? I was thinking a layer of rigid foam panel 40 mm then a layer rock wool 100 mm. If there is a 1 to 2 cm space between rockwool and sheetrock is that a problem? If XPS panels fit tight against the rafter, is there anything to be done to the joints?
1 layer of 100 mm rockwool with a space of 40 mm to the sheathing
1 layer of 140 mm rockwool
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